Activation of C3, and covalent binding of C3b to bacterial surfaces are critical to host defenses against infection. The alternative complement pathway is important in protection of the nonimmune host since, unlike the classical pathway, it usually can be activated by bacteria in the absence of specific antibody. Because children have little specific antibody, the alternative pathway is crucial for their protection. Haemophilus influenzae type b (Hib), the most common cause of bacterial meningitis in children, is resistant to alternative pathway-mediated killing. It has been established 1) that type b capsular polysaccharide (PS) is the primary virulence factor of Hib, 2) that PS is responsible for Hib resistance to serum opsonic and bactericidal activity, and 3) that specific antibody directed against PS (anti-PS) or against Hib noncapsular antigens can overcome this resistance in the presence of the classical pathway. However, it has been reported that only anti-PS can overcome Hib resistance to killing in the presence of the alternative pathway. The specific aims of this proposal are to evaluate alternative pathway resistance of Hib, and to determine how antibody directed at type b capsular PS and, possibly, noncapsular antigens, is able to overcome this resistance. These aims will be accomplished by first defining C3, factor B and factor H interactions with intact type b versus a nonencapsulated spontaneous Haemophilus (Hi) mutant. Studies will be conducted in the presence and absence of antibodies directed against capsular and noncapsular antigens to assess the role of specific antibodies. The molecular location of C3b binding on the organism, the nature of the C3b-Hib bonds, the form of C3b bound, and the effect of these factors on Hib susceptibility to functional serum opsonic and bactericidal activity will be determined. Second, C3 interactions with purified Hib capsular PS, anti-PS, and PS-(anti-PS) complexes will be investigated because it is possible that PS can bind C3 in its purified form, but not when it is attached to the surface of the organism. If so, defining differences between purified and cell-surface attached molecules will establish a foundation for developing strategies to overcome Hib alternative pathway resistance. The rationale for this proposal is that a thorough understanding of interactions between C3 and Hib eventually may permit a new approach to immunotherapy of bacterial and, potentially, viral and protozoal diseases in the form of selective activation of the alternative pathway of complement on the surface of the organism.